Despite comments elsewhere in this thread that "the web moves fast", browser changes typically involve very slow, deliberate, careful rollout plans, even for much smaller compatibility issues than this one.

I'm sure the Firefox team (and Chrome, which is pushing in the same direction) will be keeping a close eye on the progress of Let's Encrypt, and using it to set the timeframe for their proposed changes.

(Not to mention that Mozilla is a major sponsor of Let's Encrypt, so it's reasonable to expect a high degree of coordination.)